The present invention relates to a method and apparatus for continuously determining gas-carried alpha activity from the decay of thorium, uranium, plutonium and from their decay products as the difference between the total gas-carried alpha activity and the gas-carried alpha activity of the radon 220 and/or radon 222 decay chains.
In order to ascertain the alpha activity by means of the exhaust air from nuclear fuel cycle plants, two boundary values differing from each other by several orders of magnitude are considered: a higher boundary value for the proportion of the Rn-220 and/or Rn-222 decay chains and a lower boundary value for the proportion of the residual alpha emitters due to uranium, thorium, plutonium and their decay products.
Because of the differences in the boundary values it is required to monitor separately the two proportions of all of the alpha activity. The activity of the Rn-220 or of the Rn-222 decay chains is then measured separately and deducted from the total alpha activity.
Depending on the materials processed in the plants, it suffices to measure the alpha activity of only one of the two radon decay chains. When processing thorium and reprocessed uranium, the nuclides of the radon 220 decay chain by far predominate those from the radon 222 decay chain. The reverse is the case when processing natural uranium. In part, the alpha activity of both decay chains also must be determined. The radon 220 decay chain includes the alpha emitters radon 220, polonium 216, bismuth 212 and polonium 212, while the radon 222 decay chain essentially includes the alpha emitters polonium 218, polonium 214 and polonium 210.
Ordinarily the gas-carried alpha activity of the nuclides of the radon 220 and/or radon 222 decay chain is measured by means of alpha spectrometers or an alpha-beta pseudo-coincidence difference (ABPD) method.
The known alpha-spectrometric measurement methods incur the drawback that because of the energy attenuation of the alpha particle on its way to the detector, the alpha radiation of a nuclide is measured in part in a lower energy band than is actually the case. Calibration of the alpha spectrometer is made more difficult thereby and the accuracy therefore is lowered, or longer measurement times ensue. A multi-channel alpha-spectrometer entails high costs, especially when measurement filters of large diameters, for instance of 20 cm, are being used, whereby several semiconducting detectors may be required.
The ABPD meter measures alpha decays occurring shortly upon a beta decay. Because of the very short half-life values of polonium 212 and polonium 214, these nuclides allowed's the determination of the decay-product activity to radon 220 and radon 222.
However, the ABPD method incurs the drawback that measurement accuracy is degraded by the alpha decays also of other nuclides which decay during the pseudo-coincidence time interval and are included in the count. As a result, the detection limit is degraded and longer measurement times ensue. Also, an ABPD apparatus is expensive because of the relatively complex electronics.